To form a film by sputtering, ions having energy greater than a predetermined value are permitted to impinge upon a target material placed on the cathode, so that atoms or particles of the target material emitted therefrom are deposited on a semiconductor substrate to form a thin film.
As disclosed in Japanese Patent Publication No. 19319/1978, there is provided a cathode having a surface of target material, and a pair of magnetic poles are provided on a surface of the cathode on the side opposite to the target surface. Part of the lines of magnetic force appearing between the pair of magnetic poles emerges above the surface of the target material, and is curved to form a closed region as defined by arcuate lines of magnetic force and the surface of the target material. Charged particles that are generated by applying a voltage across the cathode and the anode are confined in the closed region. With the charged particles being confined in the closed region, it is made possible to form a film under a lower pressure or at a higher rate relying upon a high-density plasma than by using a two-pole sputtering apparatus.
According to this method, a-c or d-c electric power applied to the electrodes maintains the plasma and supplied energy to the ions that are incident upon the target surface from the plasma.
To increase the rate of forming film with this apparatus, it is necessary to increase the electric power that is applied to increase the plasma density. However, if the increased electric power is applied, the ions impinging upon the target surface impart increased amount of energy most of which is converted into heat when the target surface is sputtered, whereby the temperature rises. As the target temperature rises, however, the target may be ruptured or an adhered portion at the back of the target surface may be melted due to temperature stress that builds up significantly in the target. This fact imposes a limitation on the rate of forming films. Further, since the plasma region is limited to a ring form, the target is corroded in the form of a ring correspondingly. Therefore, only a portion of the target material contributes to forming the film; i.e., the utilization efficiency of the target is small.
As a method to increase the rate of forming a film under the condition where the rise of target temperature is restrained by lowering the ion impingement energy, Japanese Patent Laid-Open No. 75839/1983 proposes to employ microwaves to generate plasma. According to this method, microwaves are supplied to the cathode which confines charged particles of plasma by the lines of magnetic force, to generate plasma of a high density by the energy of microwaves, and charged particles of plasma are accelerated by the voltage applied across the anode and the cathode so as to impinge upon the target, thereby to form a film by sputtering.
With this method, the number of ions impinging upon the target increases by an amount by which the plasma density is increased, and the film is formed at a rate larger than the above-mentioned Japanese Patent Application No. 19319/1978. However, the region where the ions impinge upon the target, i.e., the region of corrosion is limited to the area where the plasma is confined in closed space that is defined by the lines of magnetic force and the target surface. That is, ions are accelerated by an electric field nearly perpendicularly to the target surface, and impinge upon the target to spring surface to cause atoms or particles of the target from the surface thereof. Therefore, the target is corroded. The region of corrosion develops only under the plasma that is generated inside the lines of magnetic force and is, hence, limited to a particular area on the target. Accordingly, if an increased electric power is applied to the cathode in order to increase the rate of forming films, the target surface is locally heated at high temperatures like in the above-mentioned apparatus, and the target is cracked. Further, utilization efficiency of the target is not enhanced, either.
Another sputtering apparatus utilizing plasma by the microwaves has been disclosed in Japanese Patent Laid-Open No. 47728/1984. According to this apparatus, the plasma generated by the microwaves is used for effecting the sputtering and for ionizing the atoms or particles emitted by the sputtering. Namely, the plasma excited and generated by the microwaves is caused to migrate by a diverging magnetic field, the electric power is applied to the neighboring cathode on which the target is placed to produce the sputtering function, and particles of the target material emitted by the sputtering function are ionized and are deposited on a predetermined substrate. Therefore, the target is corroded over nearly the whole areas being bombarded with ions. However, in order to form the film by ionizing the atoms or particles emitted from the target, the target is not opposed to the substrate, and the atoms or particles emitted from the target plunge into the plasma.
The angles of atoms or particles emitted from the target are distributed according to a cosine law, and the target is not directed to the substrate on which the film is to be formed. Therefore, the atoms or particles are directly deposited in small amounts on the substrate, i.e., the film is formed by the atoms or particles that are ionized. Hence, the rate of forming the film is dependent upon the ionization efficiency. Further, if the target having a large size is employed, there arises a problem with regard to uniformity in the plasma density distribution. That is, since the plasma has not been confined on the target surface, the density of plasma becomes high near the plasma transit window and decreases as it goes away from the plasma transit window. Therefore, the central portion of the target is corroded at an increased rate. Moreover, if an increased electric power is applied to the cathode, the impingement of ions is concentrated on the central portion of the target, giving rise to the occurrence of thermal stress. Accordingly, even in this art of forming the film, problems remain unsolved with regard to thermal stress and utilization efficiency of the target, hindering the effort for increasing the film-forming rate.